Internal combustion engine manufacturers have been developing continuously variable valve lift (CVVL) systems to actuate combustion valves (intake valves and/or exhaust valves) of an internal combustion engine in an effort to increase fuel economy, decrease emissions, and otherwise improve the performance of the internal combustion engine. These CVVL systems may be applied to only the intake valves, to only the exhaust valves, or to both the intake valves and the exhaust valves depending on the need of the internal combustion engine. CVVL systems are used to vary the magnitude of lift of the combustion valves where valve lift is commonly understood to be the distance the combustion valve is moved from its valve seat. One CVVL system is shown in United States Patent Application Publication No. US 2011/0061618 which is commonly assigned and is incorporated herein by reference in its entirety. In this CVVL system, an engine camshaft with an engine camshaft lobe is rotated about an engine camshaft axis as is customary in the internal combustion engine art. A rocker assembly receives input from the engine camshaft lobe via a roller on the rocker assembly where rotational motion of the engine camshaft lobe causes the rocker assembly to pivot in a reciprocating manner. An output portion of the rocker assembly acts on a roller of a finger follower which pivots on a hydraulic lash adjuster. When the finger follower pivots about the hydraulic lash adjuster, a combustion valve is opened and closed. In order to vary the valve lift of the combustion valve, a control shaft is provided which is rotatable about a control shaft axis by an actuator. Rotation of the control shaft about the control shaft axis changes the position of the rocker assembly which results in a change of valve lift of the combustion valve. In the event of a failure of the actuator, it may be desirable for the CVVL system to default to a predetermined valve lift which allows the internal combustion engine to start and to run satisfactorily until a repair can be made.
U.S. Pat. No. 7,886,703 teaches a CVVL system with a default mechanism for providing a default valve lift in the event of a failure of the actuator which is an electric motor. In this arrangement, rotary motion of the electric motor is converted into linear motion by a ball screw. The linear motion created by the ball screw is converted into rotational motion of the control shaft by linkage attached to the ball screw and the control shaft. The default mechanism includes two compression springs which act in opposing directions to provide a default valve lift. One drawback to this default mechanism arrangement is that the actuator must work against at least one of the compression springs over the full range of motion of the control shaft during operation which increases the capacity requirements of the actuator. Another drawback to this default mechanism is that it must be used in a system where rotational motion of the actuator is converted into linear motion.
U.S. Pat. No. 7,418,933 teaches a CVVL system with a default mechanism for providing a default valve lift in the event of a failure of the actuator which is an electric motor. In this arrangement, the electric motor has an output shaft with a driving gear which meshes with a driven gear. The driven gear is connected to a shaft of a worm gear which meshes with a sector gear of the control shaft of the CVVL system. The default mechanism includes a large diameter gear and a small diameter gear which move with the shaft of the worm gear. A first default spring surrounds the control shaft to bias the control shaft from a maximum lift position to a default position intermediate the maximum lift position and a minimum lift position. A second default spring acts on a gear set, which includes a large diameter gear and a small diameter gear, to bias the control shaft from the minimum lift position the default position. One drawback to this default mechanism arrangement is that the actuator must work against at least one of the default springs over the entire range of motion of the control shaft during operation which increases the capacity requirements of the actuator. Another drawback to this default mechanism is the cost and complexity that are added by the gears that are needed only for the second default spring to bias the control shaft from the minimum lift position to the default position.
What is needed is a CVVL system with a default mechanism which minimizes the size requirements of an actuator of the CVVL system. What is also needed is a CVVL system with a default mechanism that adds minimal components and complexity to the CVVL system.